A deadline for resolving a 12-year-old dispute over Iran's nuclear program may be extended from Monday until March because of sharp disagreements between Tehran and world powers, officials close to the talks said on Thursday.

U.S. Secretary of State John Kerry was due to meet Iranian Foreign Minister Mohammad Javad Zarif and European Union envoy Catherine Ashton to try to break the stalemate, but talk of an extension prompted calls for tougher sanctions in Washington.

The United States and its allies had hoped this week's talks in Vienna would be the culmination of months of difficult diplomacy between Iran and the United States, Britain, France, Germany, Russia and China on a comprehensive agreement.